1. Field of the Invention
This invention relates to the field of apparatus for electrostatic or high voltage spray coating. More specifically, the invention relates to voltage blocks for electrically isolating a supply of conductive coating material from a source of electric charge, for example, an electrostatic spray head.
2. Description of the Prior Art
Electrostatic spraying is employed primarily in the field of palm application. In electrostatic spraying apparatus, paint or the like is drawn from a paint pot and delivered to and through a spray nozzle. A high potential is applied to the paint in the vicinity of the spray nozzle so that charged paint is discharged from the nozzle. In a known manner, the charged paint particles are selectively attracted to the articles to be painted or coated.
Many solvent based paints have a relatively low conductivity and a correspondingly high resistivity. Solvent based paints with low conductivity are typically supplied directly from an electrically grounded palm pot to a charged spray head. Because such solvent based coatings do not provide a significant electrical path for the potential applied to the paint in the spray head, i.e., they do not create a short circuit back to the grounded paint pot, such solvent based coatings can be adequately charged at the spray head. Waterborne paints and other coatings are highly conductive, however, and they provide a path through which a charge can and does create a short circuit from a charged spray head to a paint supply container at ground potential. Thus, special equipment is needed to apply conductive coating materials such as waterborne paint electrostatically,
Because of the known hazards attributable to the use of solvent based paint systems, there is a preference for using waterborne paints and coatings, despite the need to electrically isolate the high voltage used in electrostatic spraying equipment from the paint pot from which waterborne paint is drawn. This problem has been addressed in three primary ways. In a first approach, the paint pot is electrically isolated from ground potential and is charged to substantially the same potential as the spray head so that there is no tendency for a charge to leak from one to the other or to ground. In the second and third approaches, a grounded paint pot delivers grounded conductive coating material to a spray head by way of a voltage block device. In one known voltage block device, an intermediate paint pot is filled while it is electrically isolated from the charged spray head. When filling is completed, the intermediate paint pot is electrically disconnected from the grounded paint pot and is connected to the charged spray head. In some embodiments of this type of voltage block device, a pair of intermediate paint pots are provided along with valving to alternatively connect and disconnect first one intermediate pot and then the other from the charged spray head. The unconnected intermediate paint pot is then filled before it is reconnected to the spray head. A second type of known voltage block device comprises a single intermediate paint pot and a paint feed line which includes a pneumatically operated metal tube movable between a first position in which it connects the intermediate paint pot and the grounded paint pot and a second position in which it creates an air gap of suitable length between the intermediate paint pot and the grounded paint pot. When no charge is being applied to the system, the tube is moved to the first position and the intermediate paint pot is filled. Before the system is charged, the tube is moved to the second position to electrically isolate the paint in the intermediate pot and a charge is applied to the spray head and the intermediate paint pot.
U.S. Pat. No. 3,122,320 (Beck et al.) discloses apparatus for electrically isolating a paint supply from the charge applied to an electrostatic spray head. In this apparatus, paint from a grounded paint supply is "sprayed or otherwise broken into discrete particles by [passing through a] perforated head 18" (column 1, lines 69 and 70). In other embodiments disclosed in the patent, paint at ground potential is delivered to an intermediate vessel (30 in FIGS. 2A and 2B; 31 in FIGS. 3A and 3B) which, when filled, is disconnected (and electrically isolated) from the grounded paint supply and connected to a charged paint pot 10 into which the paint is dispensed from the intermediate vessel (30 or 31).
U.S. Pat. No. 5,078,168 (Konieczynski et al.) discloses a two stage voltage block for electrically isolating an electrically grounded paint supply from a charged electrostatic spray head. Each of the two stages include shuttle devices and reservoir piston pumps. These stages are connected in series. The first stage is connected to a grounded paint supply and the second stage and the second stage is connected to the first stage and to one or more electrostatic spray heads. The first stage alternately draws conductive coating material from a grounded paint supply and delivers the conductive coating material to the second stage. The second stage alternately draws paint from the first stage and delivers it to one or more spray heads.
U.S. Pat. No. 5,1.97,676 (Konieczynski) discloses a device comprising the two stage voltage block described in Konieczynski et al., with individual voltage blocks interposed between the second stage of the Konieczynski et al. voltage block and individual spray heads.
U.S. Pat. No. 4,884,752 (Plummer) discloses a voltage block device with two chambers which are alternately connected to a grounded supply of paint for filling (while disconnected from a charged spray head) and connected to a spray head for discharging paint (while disconnected from a source of grounded paint). Supply and discharge hoses associated with the chambers are purged and dried to provide an air gap to electrically isolate the chambers.
U.S. Pat. No. 4,792,092 (Elberson et al.) discloses an improved version of the voltage block disclosed in Plummer wherein the two chambers comprise helical tubing.